The two small circuits below are simple pre-amplifiers that can easily be assembled to be used with many of your intended audio projects . Any similar transistors can be used. The RF potentiometer adjust the negative feed back used to bias to Q1 while Q2 provides for a low impedance output.

Both circuits require a minimum input of 100mV for a gain of 20 for A at 14 mA and a gain of 30 for B at 26 mA into a 5K ohms load without distortion.

 

If you are in need of something simple to check the presence of low DC voltage on your projects the next two circuits give you a choice of a buzzer (figure C) or an LED (figure B) as an indicator when voltage is detected.

 

Circuit " G " is a little gimmick that you might like to try. It is a remote control switch that can be activated only by the direct beam of a flashlight or lazer. It could be installed hidden in a dark area where you would prefer not to have a visible wall switch to activate anything you like. The phototransistor is mounted into the recess of a small tube deep enought and calibrated so that ambient light would not activate it.

The next circuit , figure " H " is a DC voltage level detector . It is straight forward in its application, adJusting the 100K potentiometer for the voltage level to be detected will activate the relay which in turn can turn on or off another devices such as a lamp, switch,circuit .